Removal of nrncore_callbacks after CoreNEURON repo merge

[pramodk]

When NEURON and CoreNEURON were separate repos, we needed callbacks / function-pointer mechanism between NEURON and CoreNEURON: See

• https://github.com/neuronsimulator/nrn/blob/d2a76abcdc5f8b8a1a687a52f350e0255d1bd817/src/nrniv/nrncore_write/callbacks/nrncore_callbacks.h
• Especially

  nrn/src/nrniv/nrncore_write/callbacks/nrncore_callbacks.h

  Line 195 in d2a76ab

  static core2nrn_callback_t cnbs[] = {

• nrn/src/nrniv/nrncore_write/callbacks/nrncore_callbacks.cpp

  Line 34 in d2a76ab

  void map_coreneuron_callbacks(void* handle) {

With NEURON and CoreNEURON in the same repo, I think all the function-pointers related logic could be removed?

[nrnhines]

That is certainly the case if it's done at link time. And no need not to link the .o files into libnrniv.so . Certainly coming when
the full integration into NEURON starts. If the wheels have -DNRN_ENABLE_CORENEURON=ON, it seems like this could
be done earlier rather than later. What is the extra memory footprint?